Cell proliferation can be restrained by limitation in the availability of proliferative stimuli, and also by the action of negative growth regulators. There is a growing list of evidence to support the involvement of paracrine growth inhibitors and intracellular growth suppressors in the regulation of cell proliferation. Representative of these classes of molecules are interferon (IFN) and nuclear phosphoproteins like retino- blastoma (RB) gene product, the two important negative growth regulators of cell growth. The focus of this grant proposal is to explore the functional relationship between IFN and RB protein, by investigating the mechanistic involvement of RB protein in the growth regulatory action of IFN-alpha. A distinctive feature of this proposal is our original hypothesis that interferons may regulate cell growth by modulating the expression of growth suppressor RB protein. Our preliminary studies suggests that growth regulation of tumor cells by IFN-alpha is closely associated with induction of expression of RB protein, and this effect of IFN-alpha is very prominent in Daudi cells which are sensitive to growth inhibition by IFN-alpha. At least, our four observations to date support the possible involvement of RB protein in the IFN-alpha-mediated growth regulation. These are: (1) IFN- alpha induced RB protein expression in Daudi IFN-sensitive cells and not in Daudi IFN-resistant cells, (2) induction of RB protein by IFN-alpha is an early event, and it precedes growth inhibition, suggesting that induction of RB may be one signal for growth inhibition, (3) IFN-alpha-induced RB protein predominantly exists as the underphosphorylated form, which is known to be expressed under situations of growth inhibition and/or differentiation, and (4) IFN-alpha does not inhibit the growth of RB minus prostate carcinoma DU-135 cells. Based on these convincing preliminary data, we wish to further explore the definitive role of this novel IFN- regulated nuclear phospho-protein in the growth regulation by IFN-alpha, and unravel the molecular mechanism of this pathway, using the Daudi cell model system. Specific Aims are to: (1) Explore mechanism of the antiproliferative action of interferon in tumor cells by investigating the modulation of expression of growth suppressor retinoblastoma gene product in IFN-alpha treated cells, (2) Explore mechanism of growth regulation by endogenous interferons by investigating the role of RB protein, (3) Explore the hypothesis of possible breakdown of novel IFN-alpha: RB pathway in IFN-alpha minus human leukemic cells by restorating regulatory growth controls through expression of functional IFN-alpha gene, and (4) Investigate the mechanisms of enhanced RB protein expression by IFN-alpha.